History

LiPo cells follow the history of lithium-ion and lithium-metal cells which underwent extensive research during the 1980s, reaching a significant milestone with Sony's first commercial cylindrical Li-ion cell in 1991. After that, other packaging forms evolved, including the pouch format now also called "LiPo".

The solid electrolyte can be typically classified as one of three types: dry SPE, gelled SPE and porous SPE. The dry SPE was the first used in prototype batteries, around 1978 by Michel Armand, Domain University,[2][3] and 1985 by ANVAR and Elf Aquitaine of France, and Hydro Quebec of Canada.[4] From 1990 several organisations like Mead and Valence in the United States and GS Yuasa in Japan developed batteries using gelled SPEs.[4] In 1996, Bellcore in the United States announced a rechargeable lithium polymer cell using porous SPE.[4]

A typical cell has four main components: positive electrode, negative electrode, separator and electrolyte. The separator itself may be a polymer, such as a microporous film of polyethylene (PE) or polypropylene (PP); thus, even when the cell has a liquid electrolyte, it will still contain a "polymer" component. In addition to this, the positive electrode can be further decomposed in three parts: the lithium-transition-metal-oxide (such as LiCoO2 or LiMn2O4), a conductive additive, and a polymer binder of poly(vinylidene fluoride) (PVdF).[5][6] The negative electrode material may have the same three parts, only with carbon replacing the lithium-metal-oxide.[5][6]

Working principle

Just as with other lithium-ion cells, LiPos work on the principle of intercalation and de-intercalation of lithium ions from a positive electrode material and a negative electrode material, with the liquid electrolyte providing a conductive medium. To prevent the electrodes from touching each other directly, a microporous separator is in between which allows only the ions and not the electrode particles to migrate from one side to the other.

Charging

The voltage of a LiPo cell depends on its chemistry and varies from about 2.7-3.0 V (discharged) to about 4.20 V (fully charged), for cells based on lithium-metal-oxides (such as LiCoO2), and around 1.8-2.0 V (discharged) to 3.6-3.8 V (charged) for those based on lithium-iron-phosphate (LiFePO4).

The exact voltage ratings should be specified in product data sheets, with the understanding that the cells should be protected by an electronic circuit that won't allow them to overcharge nor over-discharge under use.

For LiPo battery packs with cells connected in series, a specialised charger may monitor the charge on a per-cell basis so that all cells are brought to the same state of charge (SOC).

Applying pressure on LiPo cells

An experimental lithium-ion polymer battery made by Lockheed-Martin for NASA

Unlike lithium-ion cylindrical and prismatic cells, which have a rigid metal case, LiPo cells have a flexible, foil-type (polymer laminate) case, so they are relatively unconstrained. By themselves the cells are over 20% lighter than equivalent cylindrical cells of the same capacity.[7]

Being lightweight is an advantage when the application requires minimum weight, such as in the case of radio controlled models. However, it has been established that moderate pressure on the stack of layers that compose the cell results in increased capacity retention, because the contact between the components is maximised and delamination and deformation is prevented, which is associated with increase of cell impedance and degradation. [8][9]

Applications

Six edge shaped Lithium-Polymer-Battery for Underwater Vehicles made by Custom Cells Itzehoe GmbH

LiPo cells provide manufacturers with compelling advantages. They can easily produce batteries of almost any desired shape. For example, the space and weight requirements of mobile phones and notebook computers can be completely satisfied. Also, they have low-self discharge rate, which is about 5% per month.[10]

Radio controlled equipment and airsoft

LiPo batteries have just about taken over in the world of radio-controlled aircraft, radio-controlled cars and large scale model trains, where the advantages of lower weight and increased capacity and power delivery justify the price. Test reports warn of the risk of fire when the batteries are not used in accordance with the instructions.[11]

As of mid 2016, LiPo packs of 1.3 Ah exist, providing 95C continuous discharge, and short-time 190C bursts.[12] In March 2017, LiPo packs were available in various configurations, most commonly up to 6400mAh, achieving a maximum 4.2V/cell, for powering certain R/C vehicles and helicopters or drones.[13] Some test reports warn of the risk of fire when the batteries are not used in accordance with the instructions.[14]

LiPo packs also see widespread use in airsoft, where their higher discharge currents and better energy density compared to more traditional NiMH batteries has very noticeable performance gain (higher rate of fire). The high discharge currents do damage the switch contacts due to arcing (causing the contacts to oxidize and often deposit carbon), so it is advised to either use a solid-stateMOSFET switch or clean the trigger contacts regularly.

Electric vehicles

Lithium-ion cells in pouch format are being investigated to power battery electric vehicles. While it is possible to use a large number of cells of small capacity to obtain required levels of power and energy to drive a vehicle, some manufacturers and research centres are looking into large-format lithium-ion cells of capacities exceeding 50 Ah for this purpose.[citation needed] With higher energy content per cell, the number of cells and electrical connections in a battery pack would certainly decrease but the danger associated with individual cells of such high capacity might be greater.

Safety

Apple iPhone 3GS's Lithium-ion polymer battery, which has expanded due to a short circuit failure.

LiPo cells are affected by the same problems as other lithium-ion cells. This means that overcharge, over-discharge, over-temperature, short circuit, crush and nail penetration may all result in a catastrophic failure, including the pouch rupturing, the electrolyte leaking, and fire.[20]

All Li-ion cells expand at high levels of state of charge (SOC) or over-charge, due to slight vaporisation of the electrolyte. This may result in delamination, and thus bad contact of the internal layers of the cell, which in turn brings diminished reliability and overall cycle life of the cell.[8] This is very noticeable for LiPos, which can visibly inflate due to lack of a hard case to contain their expansion.

Lithium cells with solid polymer electrolyte

Cells with solid polymer electrolytes have not reached full commercialization and are still a topic of research.[citation needed] Prototype cells of this type could be considered to be between a traditional lithium-ion battery (with liquid electrolyte) and a completely plastic, solid-state lithium-ion battery.[21]

Nishi mentions that Sony started research on lithium-ion cells with gelled polymer electrolytes (GPE) in 1988, before the commercialisation of the liquid-electrolyte lithium-ion cell in 1991.[22] At that time polymer batteries were promising and it seemed polymer electrolytes would become indispensable.[23] Eventually, this type of cell went into the market in 1998.[22] However, Scrosati argues that, in the strictest sense, gelled membranes cannot be classified as "true" polymer electrolytes, but rather as hybrid systems where the liquid phases are contained within the polymer matrix.[21] Although these polymer electrolytes may be dry to the touch, they can still contain 30% to 50% liquid solvent.[24] In this regard, how to really define what a "polymer battery" is remains an open question.

Other terms used in the literature for this system include hybrid polymer electrolyte (HPE), where "hybrid" denotes the combination of the polymer matrix, the liquid solvent and the salt.[25] It was a system like this that Bellcore used to develop an early lithium-polymer cell in 1996,[26] which was called "plastic" lithium-ion cell (PLiON), and subsequently commercialised in 1999.[25]

A solid polymer electrolyte (SPE) may be, for example, a compound of lithium bis(fluorosulfonyl)imide (LiFSI) and high molecular weight poly(ethylene oxide) (PEO),[27] or a high molecular weight poly(trimethylene carbonate) (PTMC).[28]

The performance of these proposed electrolytes is usually measured in a half-cell configuration against an electrode of metallic lithium, making the system a "lithium-metal" cell, but it has also been tested with a common lithium-ion cathode material such as lithium-iron-phosphate (LiFePO4).

Other attempts to design a polymer electrolyte cell include the use of inorganicionic liquids such as 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) as a plasticizer in a microporous polymer matrix like poly(vinylidene fluoride-co-hexafluoropropylene)/poly(methyl methacrylate) (PVDF-HFP/PMMA).[29]

High-voltage LiPo cells with silicon-graphene additive

New lithium battery cell technology has introduced a silicon-graphene additive that helps to preserve the anode during discharging, thus increasing the batteries longevity and cycle-life. An inherent side-effect when operating a lithium battery above 4.2V is decreased cycle-life, with increased internal resistance.

Studies have shown that the poor capacity-retention and decreased lifespan of a lithium battery exponentially increases with corrosion of the anode, specifically when being used above 4.2V. The new silicon-graphene additive helps to preserve the anode from corrosion during use, thus preserving the Anode when operating at higher charge voltages of 4.35V or more.

The benefit of operating at the higher 4.35V voltage is an increase of about 10% energy density when compared to traditional 4.2V LiPo batteries of the same size and weight. Lithium batteries marked as "high-voltage" compatible are safe to operate at 4.35V. A standard LiPo battery should never be charged to above 4.2V as doing so can result in damage and/or fire. [30]

^Dunn, Terry (March 5, 2015). "Battery Guide: The Basics of Lithium-Polymer Batteries". Tested. Whalerock Industries. Retrieved March 15, 2017. I’ve not yet heard of a LiPo that burst into flames during storage. All of the fire incidents that I’m aware of occurred during charge or discharge of the battery. Of those cases, the majority of problems happened during charge. Of those cases, the fault usually rested with either the charger or the person who was operating the charger…but not always.

^TrakPower. Hobbico, Inc. http://www.trakpowerusa.com/batteries/lipos/index.html. Retrieved March 15, 2017. Voltages, cell counts and capacities just right for your kind of racing ... Discharge rates from 50C up to 100C ...Balanced for longer life and achieving the maximum 4.2V/cellMissing or empty |title= (help)

^Dunn, Terry (March 5, 2015). "Battery Guide: The Basics of Lithium-Polymer Batteries". Tested. Whalerock Industries. Retrieved March 15, 2017. I’ve not yet heard of a LiPo that burst into flames during storage. All of the fire incidents that I’m aware of occurred during charge or discharge of the battery. Of those cases, the majority of problems happened during charge. Of those cases, the fault usually rested with either the charger or the person who was operating the charger…but not always.